Top 3 Reasons

The most common reasons for feeling tired are about daily habits.

1. What you eat. Reaching for caffeine and sugar can backfire, leaving you more fatigued as your blood sugar levels fluctuate wildly. Instead, go for a balanced, healthy diet replete with fruits, vegetables, and lean protein. "Most people feel like they're less tired if they eat a healthy diet," says J. Fred Ralston Jr., MD, past president of the American College of Physicians. "Eating healthy also means you'll carry less weight, and obesity is a big contributor to fatigue.

2. How much you sleep. You saw this one coming, right? Many people don't get enough sleep. If you're one of them, avoid caffeine and alcohol in the hours just before bedtime, turn off the TV before bed, and keep your bedroom quiet and restful.

3. How much you exercise. This is the biggie, Ralston says. His favorite prescription for plain old tiredness is regular, vigorous exercise. Finish at least three hours before bedtime, so you have time to wind down.

If you think that exercise would just make you more tired, there's good news: Exercise breeds energy. Almost all the studies that have looked at this question have found the same thing: Sedentary people who start exercising feel much less fatigue than those who stay idle. It's one of those surprising truths: move more and you'll get more energy.

Ralston recommends getting 40 minutes of exercise at least four days a week, to get you going.

Do that, and a month from now, you should notice some improvement. Keep with it for three to six months more, and you should feel much better.

If you follow your exercise prescription for at least a month -- and you're also making enough time for sleep -- and you're still feeling lousy, look into other causes, Ralston advises.